The injuries inflicted upon a vehicle occupant in case of a vehicle crash may be classified into those due to the primary impact resulting from the collision of the vehicle with a roadside object or another vehicle and those resulting from the secondary impact due to the collision of the vehicle occupant with an on-board structure. To prevent such secondary impacts, there have been proposed various seat belt tightening systems for improving the restraining capability of the seat belts in regards to the vehicle occupants by imparting a thrust to a piston received in a cylinder in response to a signal from a sensor for detecting the deceleration at the time of a vehicle crash and rapidly winding up the seat belt with this thrust, and examples of such seat belt tightening systems for preventing forward movement of the vehicle occupant is disclosed in copending U.S. patent applications Ser. No. 07/030,630 filed Mar. 27, 1987 (European patent application No. 87.302842.9) and Ser. No. 07/132,257 filed Dec. 14, 1987 which are incorporated herein by reference.
To simplify the assembly process for such a seat belt tightening system, it is desirable to be able to construct it as a sub-assembly as long as the total mass thereof is kept within a certain limit.
However, if a seat belt tightening system incorporating a deceleration sensor is constructed as a part of a sub-assembly which includes the vehicle seat and the seat belt tightening device, there is a risk of inadvertently activating the seat belt tightening system if an excessive impact is applied to the subassembly during its transportation or at the time of mounting the seat on a vehicle body. Therefore, a considerable care will be required in the handling of the sub-assembly, and it will reduce the efficiency of the assembly work.